Spring clutch

ABSTRACT

A spring clutch is provided for which durability is improved by preventing a clutch spring from breaking due to fatigue. A cylindrical clutch spring made from a wire having a square section is mounted between a pulley and a pulley hub mounted in the pulley. An end of the clutch spring is coupled to the pulley hub. A clutch surface is formed on the inner periphery of the pulley and is formed with a large-diameter recess at a position opposite to the end portion of the clutch spring including the torque transmission end. When the pulley rotates in one direction, the clutch spring expands. At this time, the end portion of the clutch spring including the torque transmission end expands in the large-diameter recess, thus relaxing sharp torque produced in the clutch. This prevents local stress concentration on the clutch spring and breakage of the clutch spring and improves the durability of the clutch.

This application is a divisional application of application Ser. No.11/783,697, filed Apr. 11, 2007.

BACKGROUND OF THE INVENTION

The present invention relates to a spring clutch comprising a clutchspring in the form of a coil spring mounted between an input member andan output member for transmitting the rotation of the input member inone direction to the output member and cutting off the transmission ofrotation if the rotating speed of the output member exceeds the rotatingspeed of the input member.

Generally, with an engine accessory driving apparatus for transmittingthe rotation of the crankshaft of an engine to rotary shafts of engineaccessories through a belt transmission, if the engine is deceleratedquickly, a pulley mounted on the rotary shaft of each engine accessoryalso tends to decelerate quickly. Especially in the case of analternator, because the rotary shaft of the alternator has a largeinertia, it cannot decelerate quickly in response to quick decelerationof the engine, so that the pulley mounted on the rotary shaft tends tocontinue to rotate at a constant speed.

In this state, there occurs a big difference between the speed of thepulley on the crankshaft and that of the pulley on the rotary shaft ofthe alternator, so that the belt tension increases and the belt becomesliable to break.

Also, the angular speed of the crankshaft changes in one turn. Such achange in angular speed causes slip between the belt and the pulleys,thus causing fatigue of the belt and impairing its durability.

Japanese patent publication 2003-322174A proposes a clutch pulley devicewhich is free of this problem.

In this clutch pulley device, a cylindrical clutch spring made of a wirehaving a square section is mounted between a pulley and a pulley hubmounted in the pulley. The clutch spring has its outer periphery inelastic contact with a cylindrical clutch surface formed on the innerperiphery of the pulley and has one end thereof in engagement with thepulley hub. When the pulley rotates in one direction, the clutch springexpands and increases the engaging force to the clutch surface, so thatthe rotation of the pulley is transmitted to the pulley hub through theclutch spring.

When the speed of the pulley hub exceeds the speed of the pulley, theclutch spring is radially compressed, causing slip between the clutchsurface of the pulley and the clutch spring, so that the pulley canrotate freely.

With the clutch pulley device of Patent publication 2003-322174, sinceone end of the clutch spring is inserted and engaged in a spiral grooveformed in one end of the pulley hub, a gap is formed between the outerperiphery of the portion of the clutch spring and the clutch surfaceformed on the inner periphery of the pulley outside of the inlet of thespiral groove.

Therefore, in the deformation mode of the clutch spring in torquetransmission, a bend is formed at a portion of the clutch springcorresponding to the inlet of the spiral groove and stress concentratesat the bend during torque input.

As a result, the clutch spring can break at its bend due to fatigueafter repetition of loading of torque. This was a problem to be solvedto improve durability.

An object of the present invention is to provide a spring clutch whichhas its durability improved by preventing the clutch spring frombreaking due to fatigue.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a spring clutchcomprising an input member having a cylindrical clutch surface on itsinner periphery, an output member mounted in the input member so as tobe rotatable relative to the input member, and a cylindrical clutchspring mounted between the input member and the output member andincluding a torque transmission end through which torque is transmittedto the output member, wherein when the input member rotates in onedirection, the clutch spring radially expands by contact with the clutchsurface, thereby transmitting torque from the input member to the outputmember, and when the speed of the output member exceeds the speed of theinput member, the clutch spring is radially compressed, thereby cuttingoff the transmission of torque from the input member to the outputmember, and wherein the clutch surface is formed with at least onelarge-diameter recess to allow radial expansion of several turns of theclutch spring.

By forming the large-diameter recess only at a position opposite to theend portion of the clutch spring including the torque transmission end,the clutch spring can be smoothly mounted in the clutch surface withoutgetting caught.

In this arrangement, by forming the clutch surface with at least onelarge-diameter recess, when the input member rotates in one directionrelative to the output member and the clutch spring expands, severalturns of the clutch spring opposite to the large-diameter recess expand,but are not pressed against the clutch surface. Therefore, sharp torqueinput produced by the clutch is relaxed by elastic deformation of thecoil portion and no breakage of the clutch spring will occur. Thus ahighly durable spring clutch is provided.

In another embodiment of this invention, the clutch spring has at leastone small-diameter portion which is not brought into pressed engagementwith the clutch surface when the clutch spring expands.

If the small-diameter portion is formed only at a position opposite tothe torque transmission end of the clutch spring, the clutch spring canbe smoothly mounted in the clutch surface without getting caught.

In this arrangement, by providing the clutch spring with thesmall-diameter portion, when the input member rotates in one directionrelative to the output member and the clutch spring expands, thesmall-diameter portion expands, but is not pressed against the clutchsurface into engagement. Thus, sharp torque input is relaxed so thatbreakage of the clutch spring is prevented and thus the durability ofthe spring clutch can be improved.

In another arrangement, the clutch surface is formed with alarge-diameter recess at a position opposite to the end portion of theclutch spring including the torque transmission end to allow expansionof several turns of the clutch spring at its portion including thetorque transmission end, and the clutch spring comprises a clutchportion disposed radially opposite to a portion of the clutch surfaceother than the large-diameter recess, and a damper portion disposedradially opposite to the large-diameter recess, the clutch portionhaving a smaller spring rigidity than the damper portion.

In this arrangement, the clutch spring may be made from either a wirehaving a round section or a square section. But, with a clutch springmade from a wire having a round section, when the outer periphery of theclutch spring engages the clutch surface, the engaging surface would beliable to be damaged due to high surface pressure. Thus it is preferableto use a clutch spring made from a wire having a square section.

With a clutch spring made from a wire having a square section, it ispossible to make the spring rigidity of the clutch portion and that ofthe damper portion different from each other by making different thethicknesses of wires from which the clutch portion and the damperportion are made. One method of making them different is by forming acoil spring from a wire having a square section and turning the innerperiphery of the coil spring.

The spring rigidity of the clutch portion may be uniform over the entireaxial length or may increase toward the damper portion in steps orcontinuously.

If the spring rigidity of the clutch portion is made smaller than thatof the damper portion by changing the thickness of the wire by cutting,stress would concentrate at a portion where the thickness of the wirechanges sharply.

Therefore, it is possible to relax concentration of stress by changingthe spring rigidity of the clutch portion so as to gradually increasetoward the damper portion in steps or continuously.

According to another embodiment of this invention, the clutch surface isformed with a large-diameter recess at a position opposite to the endportion of the clutch spring including the torque transmission end, anda cover ring is mounted in the large-diameter recess to cover this endportion of the clutch spring. The cover ring comprises an annularportion covering the torque transmission end of the clutch spring and aplurality of hold-down claws extending axially from one end of theannular portion for restraining expansion of the clutch spring withsufficient spaces left between the adjacent ones of the hold-down clawsto allow elastic deformation of the clutch spring.

By forming the clutch surface with a large-diameter recess, when theinput member rotates in one direction relative to the output member andthe clutch spring expands, the damper portion of the clutch springexpands, but is not pressed against the clutch surface. Thus, elasticdeformation by expansion of the damper portion relaxes sharp torqueinput. Therefore, it is not probable that the clutch spring bends at thedamper portion and stress concentrates at the bend. Thus a highlydurable spring clutch can be provided.

Also, by making the spring rigidity of the clutch portion smaller thanthat of the damper portion, change in the idling torque with change inthe outer diameter of the clutch portion is small and the idling torquecan be set at a small value. Thus, wear by friction during idling andheat buildup can be suppressed and the life of the clutch function unitcan be increased.

The hold-down claws may be in a V shape having a width increasing towardthe annular portion. With this arrangement, the spaces between the clawsare the largest at their tip and decrease gradually toward the annularportion. Thus, the amount of deformation of the clutch spring betweenthe adjacent claws becomes smaller toward the torque transmission sideend of the clutch spring.

This arrangement makes it possible to increase the spring rigidity ofthe torque output end of the clutch spring gradually toward its one end,and prevent the breakage of the clutch spring effectively and relax thetorque input loaded to the clutch spring.

In this arrangement, by mounting the cover ring in the large-diameterrecess of the clutch surface so as to cover the end portion of theclutch spring including the torque transmission end and forming thecover ring with a plurality of hold-down claws, it is possible toincrease the spring rigidity of the torque output end of the clutchspring. This makes it possible to adopt as the clutch spring one madefrom a spring material which is thin, apt to deform elastically and hasgood adherence to the clutch surface, thereby improving the operabilityof the spring clutch.

Also, when the clutch spring expands into clutch engagement with theclutch surface, several turns of the clutch spring at its portionincluding the torque transmission end is prevented from expanding attheir portions opposite to the hold-down claws of the cover ring, sothat the spring rigidity increases, and at their portions opposite tothe spaces between the hold-down claws, the several turns of the clutchspring elastically deform and bulge radially outwardly, therebyrelieving sharp torque input produced by the clutch. Therefore, nobreakage of the clutch spring will occur and a highly durable springclutch can be obtained.

According to another embodiment of this invention, a radiallyelastically deformable elastic ring is mounted in the large-diameterrecess to supplement the spring rigidity of the end portion of theclutch spring including the torque transmission end.

The elastic ring may comprise a cylindrical body formed with a cut at aportion of its circumference or may comprise an annular portion having aplurality of elastic legs extending axially from one end of the annularportion with spaces between the adjacent ones of the annular legs.

A gap is formed between the outer periphery of the elastic ring and theinner periphery of the large-diameter recess and the gap is of such asize that even if the elastic ring expands under the maximum torque(allowable maximum torque to the spring clutch) loaded to the inputmember, the outer periphery of the elastic ring is kept out of contactwith the inner periphery of the large-diameter recess. This prevents thespring clutch from being loaded with shock and prevents it frombreakage.

In this arrangement, by mounting the elastic ring in the large-diameterrecess of the clutch surface, the spring rigidity of the end portion ofthe clutch spring including the torque transmission end can beincreased. This makes it possible to adopt as the clutch spring one madefrom a spring material which is thin, apt to deform elastically and hasgood adherence to the clutch surface, thereby improving the operabilityof the spring clutch.

According to a further embodiment of this invention, a ring member ismounted between the clutch surface and the clutch spring so as to getinto elastic contact with the clutch spring, and a cylindrical elasticmember is mounted between the ring member and the clutch surface so asto come into elastic contact with the outer periphery of the ring memberand the clutch surface.

The ring member may be highly rigid and not elastically deformable ormay be elastically deformable. The use of an elastically deformable ringmember increases the percentage of absorption of excessive torque byelastic deformation of both the ring member and the elastic member.Thus, an excessive torque can be absorbed more effectively in comparisonwith the arrangement in which it is absorbed by elastic deformation ofthe elastic member only.

In this embodiment, by providing the ring member between the clutchsurface and the clutch spring so as to be brought into elastic contactwith the clutch spring, it is possible to make uniform the radial forceof the clutch spring and ensure stable transmission of torque.

By providing the cylindrical elastic member between the ring member andthe clutch surface so as to get into elastic contact with the outerperiphery of the ring member and the clutch surface, if an excessivetorque (shock torque) over the transmitted torque capacity is loaded byfriction between the ring member and the elastic member, slip occurs atcontact point between the ring member and the elastic member, so thattorque transmission to the clutch spring is cut off. This preventsstress concentration to the torque transmission side end of the clutchspring and prevents breakage. Thus a highly durable spring clutch can beobtained.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and objects of the present invention will become apparentfrom the following description made with reference to the accompanyingdrawings, in which:

FIG. 1 is a vertical sectional front view of a spring clutch accordingto a first embodiment of this invention;

FIG. 2 is a sectional view taken along line II-II of FIG. 1;

FIG. 3 is a partial enlarged perspective view thereof, showing how aclutch spring is coupled with a pulley hub;

FIG. 4 is a vertical sectional front view of a spring clutch accordingto a second embodiment of the present invention;

FIG. 5 is a sectional view taken along line V-V of FIG. 4;

FIG. 6 is a vertical sectional front view of a spring clutch accordingto a third embodiment of the invention;

FIG. 7 is a sectional view taken along line VII-VII of FIG. 6;

FIG. 8 is a sectional view of a different clutch spring;

FIG. 9 is a sectional view of a still different clutch spring;

FIG. 10 is a vertical sectional front view of a spring clutch accordingto a fourth embodiment of the invention;

FIG. 11 is a sectional view taken along line XI-XI of FIG. 10;

FIG. 12 is a perspective view of a cover ring;

FIG. 13 is a perspective view of a different cover ring;

FIG. 14 is a partial sectional view of the spring clutch, showing howthe cover ring is prevented from rotating relative to the pulley hub;

FIG. 15 is a perspective view of the cover ring shown in FIG. 14;

FIG. 16 is a vertical sectional front view of a spring clutch accordingto a fifth embodiment of the invention;

FIG. 17 is a partial enlarged sectional view of the spring clutch ofFIG. 16, showing how an elastic ring is mounted;

FIG. 18 is a perspective view of an elastic ring;

FIG. 19 is a perspective view of a different elastic ring;

FIG. 20 is a perspective view of a still different elastic ring;

FIG. 21 is a vertical sectional front view of a spring clutch accordingto a sixth embodiment of the invention;

FIG. 22 is a perspective view of an elastic member; and

FIGS. 23A and 23B are perspective views of other two different ringmembers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention are described below withreference to the accompanying drawings. FIGS. 1 to 3 show a springclutch according to the first embodiment of the present invention. Asshown in FIG. 1, the spring clutch includes a pulley 1 as an inputmember, and a pulley hub 2 as an output member mounted in the pulley 1.

The pulley hub 2 comprises an output shaft 2 a made of a metal and asleeve 2 b mounted around the output shaft 2 a. The output shaft 2 a hasan opposed pair of small-diameter portions 3 and 4 at both ends thereof.Between the small-diameter portion 3 at one end of the output shaft 2 aand the corresponding end of the pulley 1, a single-seal bearing 5 ismounted to support the pulley 1 and the pulley hub 2 so as to berotatable relative to each other.

At the other end of the pulley 1, a seal 6 is mounted between the pulley1 and the output shaft 2 a and has its inner periphery in elasticcontact with the outer periphery of the small-diameter portion 4 at theother end of the output shaft 2 a.

The sleeve 2 b is molded of synthetic resin, and is pressed onto theouter shaft 2 a. The seal 6 is formed with an annular projection 7 onits inner periphery at its end facing the seal 6. The annular projection7 is axially immovably fixed between a shoulder 8 formed on the outerperiphery of the output shaft 2 a near its other end and a snap ring 9fixed on the outer periphery of the small-diameter portion 4 of theoutput shaft 2 a.

The sleeve 2 b is formed with a pair of flanges 10 and 11 on its outerperiphery at its both ends. The space between the pair of the flanges 10and 11 serves as a grease reservoir. The flanges 10 and 11 havedifferent outer diameters. The large-diameter flange 10, which islocated near the seal 6, has a slightly smaller diameter than the innerdiameter of the pulley 1. The outer peripheral surface of thelarge-diameter flange 10 serves as a radial bearing surface 12 forsupporting the pulley 1 so as to be rotatable relative to the outputshaft 2 a.

The small-diameter flange 11, which is located near the bearing 5, has alarger width than the large-diameter flange 10. As shown in FIG. 2, thesmall-diameter flange 11 is formed with a pair of diametrically opposedrecesses 13 in its side facing the bearing 5. The output shaft 2 a isformed with a pair of diametrically opposed projections 14 a and 14 b onits outer periphery at its end near the bearing 5. The projections 14 aand 14 b fit in the recesses 13, thereby preventing the sleeve 2 b fromrotating relative to the output shaft 2 a.

The pair of projections 14 a and 14 b have different lengths. The longerprojection 14 a has its outer periphery disposed radially outwardly ofthe outer periphery of the small-diameter flange 11 and located adjacentto the inner periphery of the pulley 1.

A cutout 15 is formed in the outer periphery of the small-diameterflange 11 to extend circumferentially from the recess 13 in which thelonger projection 14 a fits.

As shown in FIG. 1, a cylindrical clutch surface 16 is formed on theinner periphery of the pulley 1 between the bearing 5 and the seal 6,and a cylindrical clutch spring 17 is mounted inside the clutch surface16.

The clutch spring 17 is a coil spring made from a wire having a squaresection. In the unstressed state, the clutch spring 17 has a largerdiameter than the small-diameter portion of the clutch surface 16, andis mounted in the pulley 1 in a compressed state with the outerperiphery of its portion near the seal in elastic contact with theclutch surface 16.

As shown in FIGS. 1 to 3, a projection 19 for torque transmission is inengagement with the end of the clutch spring 17 near the bearing 5. Theprojection 19 is engaged in the cutout 15, which is formed in the outerperiphery of the sleeve 2 b. With this arrangement, the end of theclutch spring 17 near the bearing 5 serves as a torque transmission endthrough which torque is transmitted to the pulley hub 2. Its other endis a free end. The outer periphery of the free end is in elastic contactwith the cylindrical clutch surface 16. Thus, the output member and theclutch spring are rotatable indefinitely other than when the clutchspring is radially expanded by contact with the clutch surface to pressagainst the clutch surface, as discussed below.

One side surface of the projection 14 a facing the leading end surfaceof the projection 19 for torque transmission is a tapered surface 20.The leading end surface of the projection 19 is also a tapered surface21 complementary to the tapered surface 20.

As shown in FIG. 3, the end of the clutch spring 17 is brought intoengagement with the projection 19 by engaging an L-shaped engaging piece23 at the end of the clutch spring 17 in an L-shaped recess 22 formed inthe top surface of the projection 19.

As shown in FIG. 1, the clutch surface 16 is formed with alarge-diameter recess 24 at a position opposite to the end portion ofthe clutch spring 17 including the torque transmission end.

As shown in FIG. 1, a positioning ring 26 is provided between thesingle-seal bearing 5 and a shoulder 25 formed at the end of the outputshaft 2 a on its outer periphery. The positioning ring 26 has on itsouter periphery a positioning piece 27 which abuts the side of thetorque transmission projection 19. The clutch spring 17 is axiallypositioned by the positioning piece 27 and the large-diameter flange 10of the sleeve 2 b.

The pulley hub 2 is mounted on a rotary shaft of e.g. an alternator asan engine accessory so that they will not rotate relative to each other.Then, a belt is trained about the pulley 1 and the pulley on thecrankshaft to transmit the rotation of the crankshaft to the pulley 1through the belt.

In this state, when the rotation of the crankshaft is transmitted to thepulley 1 and the pulley 1 rotates in the direction shown by the arrow inFIG. 2, the clutch spring 17 radially expands by contact with the clutchsurface 16 and is pressed against the clutch surface 16.

At this time, because the clutch surface 16 is formed with thelarge-diameter recess 24 at a position opposite to the end portion ofthe clutch spring 17 including the torque transmission end, severalturns of the clutch spring 17 at this end portion radially expand butare not pressed against the inner periphery of the large-diameter recess24.

Also, with the expansion of the clutch spring 17, the torquetransmission projection 19 at the end of the clutch spring 17 smoothlyslides radially outwardly along the tapered surface 20 of the projection14 a, so that torque is transmitted to the projection 14 a and thepulley hub 2 rotates in the same direction as the pulley 1. As describedabove, the torque transmission projection 19 moves neither axially norcircumferentially, relative to the clutch spring 17. They do not moveradially relative to each other, either.

As described above, in transmitting torque from the pulley 1 to thepulley hub 2, only several turns of the clutch spring 17 at its portionincluding the torque transmission end radially expand without beingpressed against the inner periphery of the large-diameter recess 24.Therefore, sharp torque input produced at the clutch is relaxed and nobreakage of the clutch spring 17 will occur. Thus a highly durablespring clutch is provided.

Although in the embodiment of FIG. 1, the single large-diameter recess24 is provided in the clutch surface 16 at a position opposite to theend portion of the clutch spring 17 including the torque transmissionend, the position and number of such large-diameter recesses 24 are notlimited, but a plurality of axially spaced apart large-diameter recesses24 may be formed.

While torque is being transmitted from the pulley 1 to the pulley hub 2,when the speed of the pulley hub 2 exceeds the speed of the pulley 1,the torque transmission projection 19 engages the end wall of the cutout15 and the turn 17 a of the clutch spring 17 at its leading end isdragged and moves radially inwardly of the sleeve 2 b by its ownrestoring elasticity away from the clutch surface 16. Thus the clutchspring 17 is radially compressed and slips on the contact surface withthe clutch surface 16. Now the pulley 1 rotates freely and thetransmission of rotation from the pulley 1 to the pulley hub 2 is cutoff.

Even when the speed of the pulley hub 2 exceeds the speed of the pulley1, because the torque transmission projection 19 is in engagement withthe end wall of the cutout 15, projection 19 is kept close to theprojection 14 a. Thus, when the speed of the pulley 1 exceeds the speedof the pulley hub 2, the projection 19 is instantly presses against thetapered surface 20 of the projection 14 a, so that torque can betransmitted to the pulley hub 2 without delay.

FIGS. 4 and 5 show a spring clutch according to the second embodiment ofthis invention. In this embodiment, the large-diameter recess 24 of FIG.1 is omitted and the clutch surface 16 is cylindrical with a uniformdiameter over its entire axial length. Also, this embodiment differsfrom the first embodiment in that the clutch spring 17 is provided witha small-diameter portion 17 b at its portion including the torquetransmission end. Thus, elements identical to those of the firstembodiment are denoted by identical numerals and their description isomitted.

With this arrangement, in transmitting torque by the expansion of theclutch spring 17, the small-diameter coil portion 17 b only radiallyexpands and is not pressed against the clutch surface 16. Thus, like thespring clutch of FIG. 1, sharp torque input produced by the clutch isrelaxed, so that breakage of the clutch spring 17 is prevented and thusthe durability of the clutch can be improved.

Although in FIG. 4, the clutch spring 17 is provided with the singlesmall-diameter portion 17 b at its portion including the torquetransmission end, the position and number of such small-diameterportions 17 b are not limited. For example, a plurality of axiallyspaced apart small-diameter portions 17 b may be formed.

FIGS. 6 and 7 show a spring clutch according to the third embodiment ofthis invention. In this embodiment, a pulley 1 as an input member and apulley hub 2 as an output member are supported by a bearing 5 mounted atone end of the pulley 1 so as to be relatively rotatable. The pulley 1has a radially inner cylindrical clutch surface 16 which is formed witha large-diameter recess 24 at its end near the bearing 5.

A clutch spring 17 in the form of a coil spring is mounted in a radiallycompressed state in the pulley 1 and has a clutch portion 17 c at aposition opposite to the clutch surface 16 which is in elastic contactwith the clutch surface 16. The clutch spring 17 has a damper portion 17d at a position opposite to the large-diameter recess 24. The damperportion 17 d has its end portion fitted on a small-diameter springfitting surface 11 a formed on a flange 11 provided at one end of thepulley hub 2 and has its extreme end inserted and engaged in a spiralgroove 30 opening to the spring fitting surface 11 a.

A lock plate 31 fitted at the other end of the pulley hub 2 is preventedfrom coming off by a snap ring 32. The lock plate 31 and the flange 11axially position the clutch spring 17. The spring rigidity of the clutchportion 17 c of the clutch spring 17 is set to be smaller than that ofthe damper portion 17 d, which is disposed opposite to thelarge-diameter recess 24.

For setting the spring rigidity as described above, in this embodiment,the clutch portion 17 c of the clutch spring 17 is thinner than thedamper portion 17 d. Such a clutch spring can be easily made by forminga coil spring from a wire having a square section and turning one endportion of the inner periphery of the coil spring.

The clutch portion 17 c may have a uniform thickness and a uniformspring rigidity over its entire axial length, or may have a thicknessincreasing in steps toward the damper portion 17 d as shown in FIG. 8 sothat the spring rigidity changes in steps in the axial direction.Instead, as shown in FIG. 9, clutch portion 17 c may have its thicknessgradually increasing toward the damper portion 17 d so that the springrigidity continuously changes in the axial direction.

A clutch spring having the clutch portion 17 c, of which the springrigidity is lower than that of the damper portion 17 d, can also be madeby forming a spring wire having its thickness changing in the lengthdirection into a coil shape.

In this arrangement, when the pulley 1 rotates in the direction shown bythe arrow in FIG. 7, the clutch portion 17 c of the clutch spring 17radially expands by contact with the clutch surface 16 and is pressedagainst the clutch surface 16.

At this time, because the clutch surface 16 is formed with thelarge-diameter recess 24 at a position opposite to the damper portion 17d, the damper portion 17 d only radially expands without being pressedagainst the inner periphery of the large-diameter recess 24.

The expansion of the clutch portion 17 c increases a pressed contactengaging force against the clutch surface 16, so that the clutch spring17 rotates together with the pulley 1. Thus, the torque of the pulley 1is transmitted through the one end of the clutch spring 17 to the pulleyhub 2, so that the pulley hub 2 rotates in the same direction as thepulley 1.

While torque is being transmitted from the pulley 1 to the pulley hub 2,because the damper portion 17 d only radially expands without beingpressed against the inner periphery of the large-diameter recess 24,elastic deformation by expansion of the damper portion 17 d relaxessharp torque input produced by the clutch. Therefore, it is not probablethat the clutch spring 17 will bend at the damper portion 17 d withstress concentrating there. Thus a highly durable spring clutch can beprovided.

By making the spring rigidity of the clutch portion 17 c lower than thatof the damper portion 17 d, adherence to the clutch surface 16increases, and the clutch portion 17 c is radially expanded andcompressed with small torque. This improves response of the springclutch.

FIGS. 10 to 12 show a spring clutch according to the fourth embodimentof the present invention. This embodiment differs from the thirdembodiment in that the clutch spring 17 has a uniform thickness over theentire length and has its portion including the torque transmission endcovered by a cover ring 40 mounted in the large-diameter recess 24, andthat a flange 11 formed at one end of the pulley hub 2 is formed with aprojection 41 which circumferentially opposes the torque transmissionend of the clutch spring 17, and through which torque is transmitted.Thus, elements identical to those of the third embodiment are denoted byidentical numerals and their description is omitted.

As shown in FIGS. 10 and 12, the cover ring 40 comprises an annularportion 42 covering the torque transmission end of the clutch spring 17and a plurality of hold-down claws 43 extending axially from one end ofthe annular portion 42 for restricting radial expansion of the clutchspring 17 with sufficient spaces left between the adjacent hold-downclaws to allow elastic deformation of the clutch spring 17.

By providing the cover ring 40 in the large-diameter recess 24 so as tocover the end portion of the clutch spring 17 including the torquetransmission end, it is possible to increase the spring rigidity of thisportion of the clutch spring 17. This makes it possible to adopt as theclutch spring 17 one made from a spring material which is thin, apt todeform elastically and has good adherence to the clutch surface 16,thereby improving the operability of the spring clutch.

When the clutch spring 17 radially expands and engages the clutchsurface 16, the several turns of clutch spring 17 at its portionincluding the torque transmission end are prevented from expanding attheir portions opposite to the hold-down claws 43 of the cover ring 40,so that their spring rigidity increases. The several turns of the clutchspring elastically deform so as to bulge radially outwardly at theirportions opposite to the spaces between the hold-down claws, therebyrelieving sharp torque input produced by the clutch. Therefore, nobreakage of the clutch spring 17 will occur and a highly durable springclutch can be obtained.

As shown in FIG. 13, by forming a radially inwardly extending flange 44on the other side of the annular portion 42, the cover ring 40 isprevented from axially moving and producing noise because the flange 44is axially positioned by the flange 11 shown in FIG. 10 and the bearing5.

As shown in FIGS. 14 and 15, the hold-down claws 43 may be V shapedhaving a width increasing toward the annular portion 42. With thisarrangement, the spaces between the claws 43 are the largest at theirtip and decrease gradually toward the annular portion 42. Thus, theamount of deformation of the clutch spring 17 between the adjacent claws43 becomes smaller toward the torque transmission end of the clutchspring 17.

This arrangement makes it possible to increase the spring rigidity ofthe clutch spring 17 gradually toward the torque transmission end,prevent the breakage of the clutch spring effectively and relax thetorque input loaded.

Further, as shown in FIGS. 14 and 15, by forming the annular portion 42with a cutout 45 to receive the projection 41, the cover ring 40 can beprevented from turning relative to the pulley hub 2, thereby preventingthe cover ring 40 from rotating freely and producing noise.

FIGS. 16 to 18 show a spring clutch according to the fifth embodiment ofthe invention. In this embodiment, an elastic ring 50 is mounted inplace of the cover ring 40 to supplement the spring rigidity of theportion of the clutch spring 17 near the torque transmission end.

The elastic ring 50 is formed with a cut 52 at a portion of thecircumference of a cylindrical member 51. As shown in FIG. 17, betweenthe elastic ring 50 and the large-diameter recess 24, there is formed agap 6 of such a size that even if the pulley 1 is loaded with themaximum design torque (that is, maximum allowable torque to the springclutch) and the elastic ring 50 expands, the outer periphery of theelastic ring 50 does not contact the inner periphery of thelarge-diameter recess 24.

As shown in the fifth embodiment, by mounting the elastic ring 50 at theportion of the clutch spring 17 including the torque transmission end,the spring rigidity of this portion of the clutch spring 17 increases.This makes it possible to adopt as the clutch spring 17 one made from aspring material which is thin, apt to deform elastically and has goodadherence to the clutch surface 16, thereby improving the operability ofthe spring clutch.

Also, because the elastic ring 50 supplements the spring rigidity of theportion of the clutch spring 17 including the torque transmission end,it is possible to suppress the formation of a bend at this portion ofthe clutch spring 17 in transmitting torque by the expansion of theclutch spring 17. Therefore, the clutch spring 17 will hardly break dueto fatigue and a highly durable spring clutch can be obtained.

When the clutch spring 17 radially expands and engages the clutchsurface 16, the torque output end of the clutch spring 17 expands intoclose contact with the inner periphery of the elastic ring 50. As theamount of its expansion increases, the elastic ring 50 expands. Theelastic deformation by the expansion of the end portion of the clutchspring 17 including the torque transmission end and the expansion of theelastic ring 50 relaxes sharp torque input produced by the clutch andeffectively prevents the formation of a bend on the clutch spring 17.This prevents breakage of the clutch spring 17 and the pulley hub 2.

As shown in FIG. 19, the elastic ring 50 may have a plurality of elasticlegs 54 extending integrally from one end of an annular portion 53 so asto be circumferentially spaced apart from each other. As shown in FIG.20, the elastic legs 54 may have their width decreasing gradually towardtheir tip so that their spring rigidity increases gradually toward theannular portion 53. This gives a spring rigidity corresponding to theexpansion force to the end portion of the clutch spring 17 including thetorque transmission end.

Preferably, the elastic ring 50 shown in any of FIGS. 18 to 20 has acutout to receive the projection 41 formed on the flange 11, like theembodiment shown in FIGS. 14 and 15, thereby preventing the ring 50 fromrotating relative to the pulley hub 2 and producing noise.

FIGS. 21 and 22 show a spring clutch according to the sixth embodimentof the invention. In this embodiment, a ring member 60 is mountedbetween the clutch surface 16 formed on the inner periphery of thepulley 1 and the clutch spring 17 mounted in the pulley 1 so that theclutch spring 17 is brought into elastic contact with the ring member60. A cylindrical elastic member 61 is mounted between the ring member60 and the clutch surface 16 so as to be brought into elastic contactwith both the outer periphery of the ring member 60 and the clutchsurface 16.

As shown in FIG. 22, the elastic member 61 is a thin cylindrical body 62made of a metal and formed with a plurality of opposed pairs of cuts 63arranged circumferentially at equal intervals. A plurality of elasticprotrusions 64 are formed by radially inwardly bulging the portionsbetween the respective pairs of cuts 63.

The clutch spring 17 has a uniform diameter over the entire axial lengthand has its one end inserted and engaged in a spiral groove 30 formed inthe flange 11 like the arrangement shown in FIG. 6.

Otherwise, this embodiment is the same as the second embodiment shown inFIG. 4. Thus, identical parts are denoted by identical numerals andtheir description is omitted.

In the sixth embodiment, by providing the ring member 60 between theclutch surface 16 and the clutch spring 17 so as to be brought intoelastic contact with the clutch spring 17, it is possible to makeuniform the radial force of the clutch spring 17 and ensure stabletransmission of torque.

By providing the cylindrical elastic member 61 between the ring member60, which is a rigid body, and the clutch surface 16 so as to be broughtinto elastic contact with the outer periphery of the ring member 60 andthe clutch surface 16, if an excessive torque (shock torque) over thetransmittable torque capacity is loaded by friction between the ringmember 60 and the elastic member 61, slip occurs at the contact pointbetween the ring member 60 and the elastic member 61, so that torquetransmission to the clutch spring 17 is cut off. This prevents stressconcentration to the end portion of the clutch spring 17 including thetorque transmission end and thus its breakage. Thus, a highly durablespring clutch is provided.

As shown in FIG. 23A, the ring member 60 may be formed with an axial cut65 or, as shown in FIG. 23B, it may be formed with a plurality of slits66 extending axially from one end thereof so as to be elasticallydeformable. This increases the percentage of absorption of excessivetorque by elastic deformation of both the ring member 60 and the elasticmember 61. Thus, an excessive torque can be absorbed more effectively incomparison with the arrangement in which it is absorbed by elasticdeformation of the elastic member 61 only.

1. A spring clutch comprising an input member having a cylindricalclutch surface on its inner periphery, an output member mounted in saidinput member so as to be rotatable relative to said input member, and acylindrical clutch spring mounted between said input member and saidoutput member and including a torque transmission end through whichtorque is transmitted to said output member, wherein when said inputmember rotates in one direction, said clutch spring radially expands bycontact with said clutch surface, thereby transmitting torque from saidinput member to said output member, and when the speed of said outputmember exceeds the speed of said input member, said clutch spring isradially compressed, thereby cutting off the transmission of torque fromsaid input member to said output member, and wherein said clutch springhas at least one small-diameter portion which are not pressed againstsaid clutch surface when said clutch spring radially expands.
 2. Thespring clutch of claim 2 wherein said small-diameter portion is formedonly at an end portion of said clutch spring including said torquetransmission end.